Got a chance to look at this today, and, erm, well, if compatibility is the goal then it's wide of the mark I think.
Our application is ready to ship, so the number of issues under 4.0.7 is very low, and certainly nothing too serious.

Thought I'd try with the new beta since you've stated that it should be compatible, and am interested in seeing the performance improvements, especially under older IEs...

Under 4.1.0 am having lots of trouble with my custom tree (additional data being attached to the root node is getting lost, breaking my configuration and load process completely), and grids appear to not be displaying all data (can see two records coming back, but only one is in the grid, so I assume store).
No real progress with either of these yet, but will try and find out what is going on... looking at the tree first.

Some observations so far; will add more as I spot them.
  • AbstractComponent.getState used to return an empty object if there was no state, now it returns null.
    This means classes overriding the behaviour need to OR the result from calling parent with {} to ensure they can use the object.
  • Can't slice my theme; get a whole load of errors.
    Assuming it needs a new version of the SDK? If so where is it please (Windows please, my MBP is no longer my work machine)?
  • Changes to TreeStore.setRootNode appear to be causing my first problem with my tree... still investigating. Suspect it's my old mate NodeInterface, it really is the bane of my life that horrid thing!
Have removed nearly all the my "framework fixes" (Ext overrides basically), except fairly trivial ones that I have spotted have not been addressed (and are nothing to do with layout, drawing, components, elements etc), so have a fairly clean slate.

Subjectively, layout seems snappy, but of course since it's having trouble loading most of my data it's bound to be quick

More as I get it.